Costco has delayed the transition from American Express to Visa cards for its customers, a move that has seriously hurt American Express.

The Issaquah retailer signed an exclusive agreement with Citi in March 2015 that changed the company’s Costco-brand credit cards to Visa cards, ending its 16-year deal with American Express (NYSE: AXP). Costco customers will all get new cards this summer, likely in June.

American Express’ stock has fallen 36 percent, from about $82 a share before the breakup was announced, to around $52 a share early this week.

Despite that, AmEx executives said the deal with Costco (Nasdaq: COST) was hurting business. American Express CEO Kenneth Chenaultsaid in October that his company lost money every time a Costco customer swiped the card.

“The numbers didn’t add up,” he said at the time.

Costco credit cards represent about 10 percent of all American Express cardholders. That has been a big loss for the company.

It also shows the power Costco – the world’s second-largest traditional retailer – has over the companies with which it does business. The company has more than 112,000 employees and reported revenue of $116 billion last year.

New York-based American Express stock is currently the worst-performing Dow stock, down 22 percent for the year, compared to a 7 percent drop for all Dow companies. Investor Warren Buffett is the company’s largest shareholder – Berkshire Hathaway owns more than 15 percent of the company’s shares – so could push for changes in the AmEx’s leadership if things continue to slide.

AmEx said in a recent earnings report that it plans to “streamline the company and drive efficiencies” to cut $1 billion from overall costs by the end of 2017.

The company’s contract with AmEx initially was slated to end March 31, but has been extended until the Citi transfer is complete. The shift to the new credit cards has been tough on Costco, as well.

It reduced Costco’s most recent quarterly earnings by $15 million. Costco relies on membership fees as part of its revenue base. The company collected $593 million in those fees in the most recent quarter from 81.3 million member cardholders. Membership grew 1.6 percent, the slowest growth rate in several quarters.